This inventions relates to a method for the periodic determination of a quantity to be measured, above all for the continuous measurement of the mass of aerosol particles in gaseous samples, each determination of the measurement value involving the measuring of at least one measuring signal which differs from the source signal due to the influence of the quantity to be measured, and of at least one reference signal which is not subject to the influence of the quantity to be measured, after which the difference between the two signals is formed and the value of the quantity to be measured is inferred from this difference, and to a device for implementation of this method.